Overall Mortality After Diagnosis of Breast Cancer in Men vs Women

IMPORTANCE Survival differences between male and female patients with breast cancer have been reported, but the underlying factors associated with the disparity have not been fully studied. This understanding is fundamental to developing strategies for cancer treatment and survivorship care. OBJECTIVE To compare mortality between male and female patients with breast cancer and quantitatively evaluate the factors associated with sex-based disparity in mortality. DESIGN, SETTING, AND PARTICIPANTS This large, nationwide, registry-based cohort study used the National Cancer Database to identify and obtain data on patients who received a breast cancer diagnosis between January 1, 2004, and December 31, 2014. After exclusions, the final study population comprised 1816 733 patients. Statistical analyses were conducted from September 1, 2018, to January 15, 2019. MAIN OUTCOMES AND MEASURES The primary outcome was overall survival. Secondary outcomes were 3-year and 5-year mortality. Mortality differences were evaluated by Kaplan-Meier analysis. The roles of race/ethnicity, clinical characteristics, treatments, and access-to-care factors in the association between sex and mortality were estimated by nested Cox proportional hazards regression models with adjustment for age. RESULTS In total, 16 025 male (mean [SD] age, 63.3 [13.0] years) and 1800 708 female (mean [SD] age, 59.9 [13.3] years) patients with breast cancer were included in the study. Compared with female patients, male patients had higher mortality across all stages. For men, the overall survival rate was 45.8% (95% CI, 49.5-54.0; P < .001), the 3-year rate was 86.4% (95% CI, 85.9-87.0; P < .001), and the 5-year rate was 77.6% (95% CI, 76.8-78.3; P < .001). For women, the overall survival rate was 60.4% (95% Cl, 58.7-62.0; P < .001), the 3-year rate was 91.7% (95% CI, 91.7-91.8; P < .001), and the 5-year rate was 86.4% (95% Cl, 86.4-86.5; P < .001). Overall, clinical characteristics and undertreatments were associated with a 63.3% excess mortality rate for male patients. A higher proportion of excess deaths in men were explained by these factors in the first 3 years after breast cancer diagnosis (66.0%) and in all patients with early-stage cancer (30.5% for stage I and 13.6% for stage II). However, sex remained a significant factor associated with overall mortality (adjusted hazard ratio [HR], 1.19; 95% CI, 1.16-1.23) as well as mortality at 3-year (adjusted HR, 1.15; 95% CI, 1.10-1.21) and 5-year (adjusted HR, 1.19; 95% CI, 1.14-1.23) analyses, even after adjustment for clinical characteristics, treatment factors, age, race/ethnicity, and access to care. CONCLUSIONS AND RELEVANCE This study found that mortality after cancer diagnosis was higher among male patients with breast cancer compared with their female counterparts. Such disparity appeared to persist after accounting for clinical characteristics, treatment factors, and access to care, suggesting that other factors (particularly additional biological attributes, treatment compliance, and lifestyle factors) should be identified to help in eliminating this disparity.